Pulse generation



May 10, 1949. J. F. GORDON PULSE GEFERATION 2 Shams-Sheet 1 Filed Jan, 18. 1946 7 4 c m m w l 4s w an 8 4 all: IT R U. U U C A M O, r T R M B mw n o W. Ru D R R m m 2 k 5 W A D M m o M PULSE GENERATOR CRYSTAL OSCILLATOR SYNCH. PULSE INPUT 3mm JAMES F. Gompow AUDIO 1 10, W49. J. F. GORDON 2,470,028

PULSE GENERATION Filed Jan. 18, 1946 2 Sheets-Sheet 2 w 4 5 1:91- (a) QI CPU w MMAMMMMMM g vvvvvvvvvvvvvvvv 3mm JAMES F. GORDON Maw Patented May 10, 1949 PULSE GENERATION James F. Gordon, Baltimore, Md., assignor to Bendix Aviation Corporation, Towson, Md., a corporation of Delaware AppiicationJanuary 18, 1946, Serial No. 641,945

S PATENT OFFICE in the communication art, whenever it becomes necessary to vary one of the characteristics of a periodic wave, as for various types of modulation processes. Application of the invention to the phase modulation of a stabilized oscillation has proved particularly useful, since a-relatively wide phase deviation may be accomplished without reaction on the frequency stabilizing apparatus. In order to develop the merits of the invention, this particular application will be considered first.-

The conventional method of generating phase modulated waves makes useof a process which varies the phase of a current derived from a fixed frequency source by an amount directly proportional to the amplitude of the modulating current and inversely proportional to its frequency. The resultant phase shift is multiplied by a series of frequency multiplications to the degree necessary to produce the desired frequency change.

With the existing systems it has been difiicult to obtain phase modulation exceeding thirty degrees without serious distortion, andto maintain the necessary stability of the mean or unmodulated carrier frequency. This is particularly true in the design of high frequency transmitters, which utilize a low-frequency crystal controlled oscillator, since undesired changes in the mean oscillator frequency are multiplied'by the same factor as the desired step-up of the mean frequency.

One of the objects of the invention is to provide a method and means for generating unidirectional pulses and modifying their spacing with respect to time in accordance with a control voltage. I

Another object of the invention is to provide a method and means for generating a radio wave and modifying its phase in accordance with the signal to be transmitted.

A. further object of the invention is to provide a circuit of the multivibrator type for modifying the phase of a radio wave in accordance with the signal to be transmitted.

Still another object of the invention is to provide a method and means for phase modulatin a radio wave over a greater angle of deviation than is at present obtainable by conventional transmission apparatus.

4 Claims. (01. 332-14) Another object of the invention is to provide a method and means for producing wide frequency swings of a phase-modulated radio wave, while at the same time maintaining a high degree of stability of the mean frequency. I

A further object of the invention is to provide a method and means for producing wide frequency swings of a phase-modulated radio wave, while keeping'distortion at a. low level.

Still another object of the invention is to provide a method and means for producing radio frequency pulses the positions of which with respect to time are modified in accordance with the signal to be transmitted.

With these and other objects in view, the invention consists of certain novel details of construction and combination of parts, hereinafter fully described and claimed, it being understood that various modifications may be resorted to within the scope of the appended claims without departing from the spirit or sacrificing any of the advantages of the invention.

Other objects and advantages of the invention will become apparent from a consideration of the following specification taken in conjunction with the drawings, in which:

Figure 1 is a block diagram of a phase modulated transmitter system in which the invention is used;

Figure 2 is a schematic diagram of a conventional multivibrator circuit;

Figure 3 is a, schematic diagram of a multivibrator circuit, illustrating one mode of application of the invention;

Figure 4 is a schematic diagram of a multivibrator circuit, illustrating another mode of application of the invention;

Figure 5 is a graph illustrating the rectangular square wave generated by a relaxation oscillator whose coupling time constants are identical and the resulting symmetrical pulse series after dif-, ferentiation;

Figures 6 and 7 are graphs showing rectangular square waves generated by a relaxation oscillator whose coupling time constants are not identical and the resulting pulse series after differentiation; and,

Figure 8 shows a. series of graphs (a) to (e) illustrating in a time related manner various wave forms occurring in the system of Figure 1, and a graph (f) illustrating in a, like manner the output wave formof a pulse generator triggered by the pulses of graph .(d)

The invention involves modulation of the 5 square wave forms comprising the output of a for a shorter period of time than V2.

Y ings, Figure 1 shows the general arrangement of a phase modulation transmitting system embodying the invention.

The system shown in Figure 1 includes a multivibrator I2, the output of which would normally be a uniform rectangular wave form such as indicated by the graph 43. However, by the application of a modulating voltage from the modulator I5, in a manner to be described later, the relative durations of the positive and negative excursions of each cycle are altered as a function of the modulating voltage. The nature of the than that of the Re, C1, R3 combination, then resultant wave form is indicated by the graph 44. The wave form of the modulating voltage producing the output 44 is indicated by graph 45.

The output 44 is differentiated and the positive pulses of the resulting wave form are clipped in the portion of the system represented by the block l3. The result is a train of negative pulses,

as indicated by graph 46, the positions of which are varied about a means as a function of the modulating voltage. The polarity of the pulse train may, of course, be inverted if desired to fit the requirements of use.

These pulses are usable for various purposes, as pointed out above, being indicated vhere as applied, to a class C radio frequency amplifier l4. If this amplifier be provided with a plate c ircuit resonant at the multivibrator frequency or any multiple thereof, the output of the amplifier will be a sine wave phase modulated in accordance with the modulating voltage applied to the multivibrator. 1

Means for stabilizing the frequency of the multivibrator are shown as comprising a crystal oscillator I 0, the output of which is indicated by graph 4|, triggering a pulse generator H to proin Figure 2. The circuit and the interaction of its components being well known will not be described in detail. The normal output of the circuit is a square Wave such as those shown in Figures 5a, 6a, and 7a. The degree of symmetry of the output wave form depends upon the values of the condensers and resistors shown. If the time constants of the combination R6, C1, B3 and the combination R7, C2, R11 are identical, the wave form will be symmetrical as shown by graph (a) of Figure 5. However, if for example, G1 has more capacitance than C2 or if R3 has a higher resistance than R2, then V1 will have a longer conducting period than V2. Thus V1 will be cut off The'wave form of the output at the anode of V1 will then appear as in graph (a) of Figure 6.

If conditions are reversed so that the time constant of the R7, C2, R2 combination is greater V1 will be cut off for a'longer period of time than V1 and the output at the anode of V1 will appear as in graph (a) of Figure '7.

A similar variation in the relative cut-off time of the two tubes may be obtained by the application of a small variable voltage to the grid of either of the tubes. Thus the application of a modulating voltage as indicated in Figure 1 to the grid of either tube will cause the relative cutoff periods of the two tubes to vary in accordance therewith. By this means it is possible to vary the switch over time between relatively wide limits.

By differentiating the square wave form, a train of pulses of alternating polarity is obtained, positive pulses ll marking the leading edges and negative pulses 12 marking the trailing edges of the positive excursions of the wave form. The positions of the positive pulses are unaffected by the modulating voltage, but the positions of the negative pulses vary in accordance with the modulating voltage as indicated in the lower grap (b) of Figures 5, Sand 7.

The elimination of the positive pulses by clipping or other means leaves available a train of position modulated negative pulses for use as desired.

It should be noted that, while in this description the modulation of the output wave form of the relaxation circuit is consistently spoken of as modulation of the trailing edge of the positive excursions, the same modulation may equally well be referred to as applying to the leading edges of the negative excursions. Furthermore, if the point at which the output is taken is shifted from the anode of one tubeto the other, the wave form will be inverted, and modulation, for example, of the trailing edges of the positive excursions will become modulation, in inverted phase, of the leading edges of the positive excursions.

The invention may employ multivibrator circuits of either the balanced or unbalanced types as will be described hereinafter.

Referring now to Figure 3, there is shown a multivibrator circuit of the unbalanced type employed in accordance with the invention. This type of multivibrator is normally in a state of equilibrium with one tube conducting and the other out ofi. Upon receipt of a triggering pulse of the proper polarity, the circuit is driven through one complete cycle of operation, returning again to the original state at the termination of the cycle. If synchronizin pulses are applied at a recurrence rate which is comparable to the natural frequency of the circuit, the end of each cycle can be made to coincide exactly with the succeeding synchronizing pulse and the frequency of the multivibrator will be locked to'that of the synchronizing voltage.

The multivibrator 20 comprises a pair of electron discharge tubes 2I and 22. Their anodes are connected together through resistor 23, and connected to a power source 24 through the anode resistors 25 and 26 respectively. The control elec trode 21 of tube 2| is joined to the anode of tube 22 through condenser 28 and the variable resistor 29, which are connected in parallel. The control electrode 30 of tube 22 is connected through condenser 3| to the anode of tube 2|. The grid leak 32 grounds the control electrode 21 of tube 2|. The variable grid leak 33 connects the control electrode 30 of tube 22 to the cathode of the tube 25. The modulation voltage is applied to the grid 88 of tube 22 from source 40 through capacitor 38 and inductance 31. The multivibrator output taken from the anode of tube 2| is supplied to the differentiating output circuit l9, consisting of capacitor 38 and the grounded resistor 38, and from there to the next stage of the transmitter system.

The negative synchronizing signal applied to the grid 21 of the multivibrator tube 2| effects a decrease in the anode current of tube 2| causing its anode voltage to increase, and by means of capacitor 3| the grid 30 of tube 22 (which is normally biased to or beyond cut-off) is driven positive, causing the anode current of tube 22 to flow. This anode current in tube 22 passes a negative signal through capacitor 28 and the parallel variable resistance 29 to grid 21 of tube 2|, causing the anode current of tube 2| to further decrease. This regenerative action causes the anode current in tube 22 to rapidly reach a max imum, while the anode current in tube 2| is cut ofi. This circuit condition remains until the charge on the condenser 28 has leaked away through resistors 32 and 29 sufiiciently to permit anode current to flow once more in tube 2|. When this occurs the regenerative effect again takes place, but in the reverse direction. In order to allow the synchronizing pulses to control the operation of the multivibrator, its circuit is designed to oscillate at a frequency slightly less than the recurrence frequency of said synchronizing pulses. Thus, the multivibrator output frequency is identical to the initiating pulse repetition rate and is permanently locked to the frequency of the crystal oscillator. The application of control voltage from modulation source 40 to the grid 30 of tube 22 will cause 'the relative duration of the excursions of opposite polarity to vary in successive cycles of the multivibrator output in accordance with the amplitude variations of the control voltage.

The multivibrator output is applied to the differentiating output circuit l8, whose output will consist of a series of alternating positive and negative pulses, the equidistant positive pulses being time-position controlled by the stabilized oscillation of crystal oscillator l0, and the negative pulses by the modulation voltage.

The pulses are then fed through a clipper circuit, which clips the positive pulses, the remaining position modulated negative pulses being available for use.

In the particular mode of application, illustrated in Figure 1, the remaining negative pulses 12 are inverted and used to drive the grid of a class-C amplifier, the plate circuit of which is resonant at a frequency which corresponds to the repetition rate of the pulses driving the grid or a multiple thereof. The effects are shown in graphs (a) to (e) of Figure .8. Due to the initiating grid pulses 12 as shown in graph ((1) and the inertia effect of the plate circuit, a sine wave 13 as shown in graph (b) will be generated in the plate circuit. As a result of the effect of the control voltage 14 (graph on the multivibrator output, changes in timing of the grid pulses 12 will occur as indicated by the solid line pips of graph (d) and will result in phase deviations of this sine wave, as shown by the solid line wave form of graph (e). The dotted line pips and sine wave form of graphs (d) and (e) are replicasof graphs (a) and (b) included for reference. These phase deviations will exactly follow the circuit of the multivibrator. The mean f requency of the R.-F. energy generated in the plate circuit of the amplifier will be identical with the frequency of the crystal oscillator controlling the pulse repetition rate at which the multivibrator is actuated, or a multiple thereof. After multiplication to the required transmission frequency, the phase modulated radio wave can be radiated. Figure 4 illustrates the employment of a multivibrator circuit of the balanced type in accordance with the invention. This type of multivibrator is normally free-running but for precise frequency control may be driven by a synchronizing arrangement such as shown in Figure 1. The multivibrator 50 comprises a pair The cathodes of tubes 5| and 52 are connected together and grounded through resistor 55. and the secondary of transformer 48, and due to the voltage drop across these two elements the cathodes of both tubes are positive with respect to ground. Positive grid bias to each tube is supplied from the power source 53. The grid bias to tube 5| is controlled by the voltage divider network, consisting of the variable resistor 56 and the grounded resistor 51, and the grid bias to tube 52 is controlled by the voltage divider network consisting of resistor 58, the potentiometer 59, the grounded resistor 66, and the shunted resistors GI, 64 and 55. Resistor 6| prevents the grid circuit of tube 52 from being unduly loaded when potentiometer 59 is adjusted. The control electrode 62 of tube 52, grounded through grid leak resistor 64, and resistor 55, is connected through capacitor 63 to the anode of tube 5|. Multivibrator coupling between the two tubes is by means of their common cathode resistor 55 and the secondary of transformer 48.

Synchronizing pulse signals are supplied to the grid 65 of tube 5| through capacitor 68. The modulation voltage is applied to the primary of transformer 48, whose secondary is connected between the grid 62 and the cathode of tube 52, v

the modulation voltage thus appearing between grid and cathode of both tubes. Capacitor 68 which parallels this secondary is a by-pass condenser at the multivibrator frequency. The impedance of the secondary of transformer 48 is kept very low. It thus prevents voltage variations from extraneous sources, such as filament A.-C. hum, etc., fromappearing between cathode and grid of tube 52 in any appreciable magnitude where they would cause undesirable modulation of the circuit.

The anode current through tube52 will now vary in accordance with the audio potentials applied to the transformer primary, This variation in plate current of tube 52 will cause the cathode of tube 52 to exhibit a proportional variation in voltage between cathode and ground. Since the cathodes of tubes 5| and 52 are connected together. this same voltage variation will take place between the cathode of tube 5| and ground, and this voltage variation controls the time at which tube 5| starts conducting after having been cut off by the negative synchronizing pulse. The multivibrator output is supplied to the differentiating output circuit 49, consisting of capacitor 69 and the grounded resistance 10, and from there to the next stage of the system.

The circuit of Figure 4 is arranged to minimize the introductionv of distortion during modulation.

changes of the time consta t of the second tube 76 The lar r p r of the distortion to be encounlarge, causing operation to take place on a very short and therefore practically straight portion of the condenser discharge characteristic. The resistance of resistor 64 is very low (preferably around 500 ohms or less) and the capacity of con-' denser 63 being large, distortion due to the flow of grid current isthus kept to a very low value.

The possibility of synchronizing pulses feeding through {the multivibrator circuit to appear in the output of the system is minimized by the fact that synchronization is secured by the application of negative pulses to the grid of tube These pulses cut off tube 5|, initiating the positive excursion of the output wave form derived from the anode of'that tube. This wave form then being differentiated and the positive pips of the differential wave being clipped, any carry through of synchronizing pulses is eliminated.

Among the many possible uses of the pulses a control electrode of'the other; means stabiliz-" ing the recurrence frequency of said square wave output; a source of modulating voltage; means varying the control electrode potential of one of said electron discharge tubes in accordance with said modulation voltage; means differentiating the output of said relaxation oscillator, thus producing two alternating series of energy pulses, one of said series being of uniform spacing and the other position modulated as a function of said modulating voltage; means eliminating said series'of uniformly spaced pulses; means forgencrating radio frequency waves, said means having a natural frequency comparable to the recurrence frequency of said relaxation oscillator output and means synchronizin the output of said radio frequency wave generating means with the .re-

generated in accordance with the invention is their application as triggering pulses to the pulse oscillator of an R.-F. pulse transmitting circuit. This will result in the positive modulation of the transmitted pulses in accordance with the requirements of pulse communication systems. Figure 8' illustrates in the time related graphs (d) and (f) the wave forms pertaining to this manner of employing the invention. The pulses 12 of graph (d) here control the production of the pulses 15 of R.-F. energy, shown in graph (1'), which are position modulated in the same manner.

It will be obvious thatmany changes and modificationsmay be made in the invention without departing from its spirit as described in the foregoing description and in the appended claims.

What is claimed is:

1. In a system for generating position modulated energy pulses, the combination which comprises: a relaxation oscillator generating a square wave output, said relaxation oscillator comprising two electron discharge tubes, the anode of each of which is capacitively coupled to a control electrode of the other; means stabilizing the recurrence frequency of said square wave output by the application of synchronizin pulses to the control electrode of one of said electron discharge tubes; a source of modulating voltage; meansvarying the control electrode potential of the other of said electron discharge tubes in accordance with said modulation voltage; means differentiating maining series of energy pulses.

3. In a system for transmitting phase modulated oscillatory currents, the combination which comprises: a relaxation oscillator generating a square wave output, said relaxation oscillator comprising two electron discharge tubes; means stabilizing the recurrence frequency of said square wave output; a source of modulating voltage; means varying the rid potential of one of said electron discharge tubes in accordance with said modulation voltage; means differentiating the output of saidrelaxation oscillator, thus produc-- ing two alternating series of energy pulses, one of said series'bein'g of uniform spacing and the other position modulated as a function of said modulating voltage; means eliminating said series of uniformly spaced pulses; means for generating pulses of radio frequency oscillations and means for keying said pulse generating means by the remaining series of energy pulses.

4. In asystem for generating position modulated energy pulses, the'comb ination which comprises: a relaxation oscillator generating a square wave output, saidrelaxation oscillator comprising two electron discharge tubes, each having at least an anode, a cathode and a control electrode; means applying a synchronizing voltage to the control electrode of one of said tubes to stabilize the recurrence frequency of said square wave output; a source of modulating voltage; means applying said modulating voltage to the control electrode of the other of said tubes; means differen tiating the output of said relaxation oscillator, thus producing two alternating series of energy pulses, one of said series being of uniform spacing as determined by the application of said syn chronizing voltage and the other position modulated as a function of said modulating voltage;

and means eliminating said uniformly spaced pulses.

JAMES F. GORDON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

